The invention is directed to hybrid yarns which include natural fibers and thermoplastic fibers, reinforcing fabrics made from such yarns, and reinforced composites made from such yarns and fabrics. The yarns and reinforcing fabrics are particularly useful for low to medium strength applications. The yarns and reinforcing fabrics of the invention are environmentally-friendly alternative to fiberglass based composite reinforcements. The significant advantages of the invention include the use of natural plant bast fibers in combination with thermoplastic fibers which provide thermoformable properties for the reinforcing yarn and fabric of the invention which properties are effective to permit efficient shaping into a fibre preform.
A number of different materials, such as organic and inorganic fibers, have been used to make composite reinforcements, particularly reinforcements for low and medium strength applications. Inorganic fibers include glass and carbon filaments, filaments of metals or metal alloys such as steel, aluminum or tungsten; non metals such as boron; or metal or nonmetal oxides, carbides or nitrides such as aluminum oxide, zirconium oxide, boron nitride, boron carbide or silicon carbide, ceramic filaments, filaments of slag, stone or quartz. Organic fibers include aramid, nylon, polypropylene, polyethylene, polyester and natural fibers, such as cotton and wood.
Traditionally, fiberglass has been the most popular material for almost any composite reinforcement application. Fiberglass has unique combination of versatility and strength that made this reinforcement a material of choice for more than 50% of all composite articles manufactured in the year 2000. Nylon, polyester, and polypropylene fibers are another composite reinforcement alternative. They have been used extensively for low and medium strength composite reinforcement applications. Despite their good availability fiberglass, nylon, polyester, and polypropylene fibers have significant disadvantages, including high prices tied to crude oil prices. All of these materials pressure the environment because they are not necessarily renewable, do not biodegrade and generate significant Green House Gases emission upon manufacture and/or destruction. Key disadvantages of fiberglass also include the worker unfriendly nature of the material (fiberglass is an irritant), its fragility which makes it difficult to process; and finally, its density (natural fibers have specific density that is 40% less than density of fiberglass).
Hybrid yarns from non-thermoplastic reinforcement filaments (e.g. aramid, glass or carbon fiber) and thermoplastic filaments (e.g. polyester fiber) are well known. For instance, the patent applications EP-A-0,156,599; EP-A-0,156,600; EP-A-0,351,201 and EP-A-0,378,381 as well as Japanese Publication JP-A-04/353,525 and U.S. Pat. No. 5,792,555 consider hybrid yarns made of non-thermoplastic fibers (e.g. glass or aramid filaments or rovings) and thermoplastic fibers (e.g. polyester or PET filaments or rovings). Thermoformable textile materials (e.g. plain weave fabrics) are made from thermoformable hybrid yarns having high melting point and non-melting filament or fibers. These textile materials can be converted into fiber reinforced, stiff thermoplastic sheets that may be used for different structural applications.
Various methods of producing fiber reinforced thermoplastic sheets are described in Chemiefasern/Textiltechnik, volume 39/91 (1989) pages T185 to T187, T224 to T228 and T236 to T240. Processes are described which start with a woven mat composed of hybrid yarns. The advantage of these techniques are a mixing ratio of reinforcing and matrix fibers that can be very precisely controlled, as well as the drapability of the textile materials which makes it easy to process the material by compression moulding (Chemiefasern/Textiltechnik, volume 39/91 (1989), page T186).
EP-A-0,268,838 describes reinforcing textile material a layer of longitudinal threads and a layer of transverse threads, which are not interwoven. One of the plies of threads has a significantly higher heat shrinkage capacity than the other. Auxiliary threads provide cohesion. These auxiliary threads do not tightly bind the layers of the reinforcing threads together, but rather loosely fix them to one another so that they can move relative to one another.
DE-A-4,042,063 describes making easily deformed reinforcing layers. Longitudinal heat-shrinking and auxiliary threads are incorporated into a sheet material intended for use as textile reinforcement. Heating causes the textile material to contract as some extent, so that the reinforcing threads are held in a wavy state or in a loose looping.
U.S. Pat. No. 6,51,313 describes yarn that is formed from non-twisted discontinuous parallel fibers held together by a covering yarn of sacrificial material wound around the fibers. The fibers comprise an intimate mixture of fibers of at least two different types: 1) carbon fibers or pre-oxidized polyacrylonitrile based carbon precursor fibers, 2) anisotropic or isotropic pitch based carbon precursor fibers, 3) phenolic or cellulosic based carbon precursor fibers, and 4) ceramic fibers or ceramic precursor fibers. In a carbon state, the mixture of fibers comprises at least 15% by weight of high strength fibers having a tensile strength of at least 1500 Mpa and a modulus of at least 150 Mpa, and at least 15% by weight of fibers with a low Young""s modulus of at most 100 GPa.
DE-A-3,408,769 discloses a process for producing shaped fiber reinforced articles from thermoplastic material by using flexible textile structures consisting of substantially unidirectional aligned reinforcing fibers and a matrix constructed from thermoplastic yarns or fibers. Final shaping of a composite takes place after passing heated dies where virtually all of the thermoplastic fibers melt and bind the reinforcement.
This invention relates to hybrid yarns which include natural bast fibers, composite reinforcements made from such yarns, and a process for making such yarns. The composite reinforcements are particularly suitable for low to medium strength composite reinforcement applications. The yarns and reinforcement of the invention advantageously include plant fibers that otherwise would be burned on the field and contribute to the Green House Gases emission.
The hybrid yarns of the invention comprise short staple natural plant bast fibers and thermoplastic matrix filaments which are effective for making yarns having a tenacity of at least about 0.8 grams/Denier and a Young""s tensile modulus of at least about 6 g/Denier. Further the hybrid yarns of the present invention are capable of permanent deformation. The deformation property provides a unique deep-draw characteristic to the open textile sheet materials produced from the yarn. The reinforcement fabric of the invention is an open thermoformable mat that is capable of being used for manufacturing reinforced composite articles which are produced by deforming the thermoformable textile sheet-like mats of the invention. The open thermoformable woven mat can be uni- or multidirectionally placed to provide an article having an adjustable high strength in two or more directions.
The hybrid yarns of the invention include at least two groups which are twisted together to form the yarn. The first group includes plant bast fibers having a tenacity of at least about 1.5 grams/Denier, a breaking elongation of from about 1 to about 20% and a crimp of from about 5 to about 80%. The second group comprises at least one thermoplastic filament having a melting point of at least about 10xc2x0 C. and below the thermal decomposition point of the plant bast fibers.
Almost any natural bast fiber such as non-oil seed plant bast fibers can be used in the invention. Jute, flax, sisal, ramie, hemp and kenaf can be successfully processed into the hybrid yarn and composite reinforcement of the invention. The plants from which the bast fibers used in the invention come have bast fibers separable from the shives in the stem of the plant. As used herein, xe2x80x9cbastxe2x80x9d refers to those fibers from the phloem region. xe2x80x9cShivesxe2x80x9d refers to the core tissue particles that remain after the bast fibers are separated from the plant stem. The thermoplastic fibers or filaments (e.g. polypropylene or polyester) serve as binder for the composite and also help to process the hybrid yarn on textile equipment. The yarn is processed by short staple ring spinning from chopped bast fibers with an average fiber length about 15-75 mm and thermoplastic filaments. The yarn may be woven into a large variety of textile products, particularly into open mat type products; and more particularly into open mat products with openings that have an area in a range of 0.2 to about 100 mm2. The significant advantage of the natural fiber based yarns over fiberglass, carbon and other stiff high performance fibers is the ease of processing natural fiber based yarns into a variety of textile products cost effectively on a very large scale.
The bast fibers of the present invention are blended with a hydrophobic lubricant and antistat during blending of the bast fibers with the thermoplastic filaments and prior to carding. Bast fibers are treated with an amount of hydrophobic lubricant and antistat that is effective for increasing the affinity of the bast fibers to the surface of the thermoplastic filaments. The hybrid yarn will include about 0.1% to about 0.5% of hydrophobic lubricant, based on the dry weight of the fibers, and from about 0.1% to about 1% of antistat, based on the dry weight of the fibers. Preferably, the hybrid yarn will include about 0.2% to about 0.3% hydrophobic lubricant, based on the dry weight of the fibers, and a weight ratio of hydrophobic lubricant to antistat of about 80 to 20. A hybrid yarn of the invention having a moisture content of about 12% will have at least about 0.3% by weight hydrophobic lubricant and at least about 0.2% by weight antistat.
The hydrophobic lubricant of the invention includes compositions that contain nonionic hydrocarbon surfactants and lubricant bases that include alkyl phosphate esters, alkyl esters of fatty acids, polyoxyethylene lauryl ether and polyoxyethylene tridecyl ether blended in an inert carrier. Antistats of the present invention include antistat compositions having at least one neutralized C3-C12 alkyl or alkenyl phosphate alkali metal or alkali earth metal and a solubilizer.
The surface characteristics of the bast fibers are enhanced such that they are effective for spinning when treated as described above with hydrophobic lubricant and antistat.
The reinforced composites of the invention include either the yarns or the woven open mats of the invention. A yarn is formed from short staple natural fibers and thermoplastic filaments using ring spinning technology. The combination of natural and thermoplastic fibers provides prepreg properties for the reinforcement. The mixture of fibers comprises at least 15% by weight of natural fibers and at least 1% by weight of thermoplastic fibers.
It has been found that yarns in the prior art have significant disadvantage for low to medium strength composite reinforcement applications. Known reinforcements are designed for high performance applications and they are too expensive for broad use in cost sensitive applications like construction materials and interior trim automotive parts. Another disadvantage of reinforcing materials described in the previous art is difficulty in handling of these materials due to their irritant nature (e.g. fiberglass and carbon fibers).